Heretofore, various means have been employed in conjunction with the passage of exhaust gases from a combustion chamber to provide for the efficient transfer of heat to separately partitioned off air chambers which pass through and adjacent such exhaust passages. The main objective, heretofore, was to provide for the most efficient transfer by conduction of heat from the exhaust gases passing through the exhaust passages and through the walls thereof and with respect to fresh air passing through a transversely extending air chamber.
Examples of earlier efforts in this direction are shown in U.S. Pat. No. 1,871,322 and U.S. Pat. No. 3,124,197. Additional examples of earlier efforts to accomplish heat exchange are shown in the following U.S. Pat. Nos.: 2,102,727; 101,923; 387,715; 2,307,600; 1,984,949; 1,161,855.